Casting Thin-Walled Parts

Making molds from models with thin walls, such as this model of a computer mouse, follows a similar process as the one used in our complex mass casting mold. In this example, our model was machined from Freeman Machinable wax, a durable, economical, hard wax material that is easy to machine and requires no sealing or releasing to produce accurate silicone rubber, epoxy, or urethane duplications.

We’ve constructed a mold box from wood, ensuring that our model will be surrounded by at least 1/2 an inch of silicone rubber. For larger models, we would allow for a larger margin of silicone rubber.

Next, we apply Plast-Econ modeling clay to the bottom of our model. After pressing the model to adhere it to the mold board, we clean up the edges of the model with a fillet shaper. Then we apply a light coat of wax release to remove any remaining excess clay.

For this mold, we’re going to use four small strips of half-round pattern wax to provide the registration. Each piece is being adhered to the mold board with our Plast-Econ modeling clay.

Next, we assemble our mold box and then we mix our Rhodia V-330 silicone rubber and CA 45 catalyst. We’ve chosen the more flexible V-330 because it will be easier to demold from our wax model.

The first half of our mold is now ready to be poured and left alone to cure overnight.

The next day, we remove the mold board to reveal the underside of our model. Very carefully, we begin removing the clay that adhered our model to the mold board.

Next, because this will be a closed two-part mold, we have to create a system to vent any air that may become trapped, causing defects in our part. In addition to vertical vents, which we will apply later, we are applying clear tape around the edges of our model. This will create a uniform void around our model to permit easy escape of any unwanted air that our vertical vents cannot vent alone.

Next, we apply a coat of petroleum on the cured silicone rubber parting line. This is done because the liquid silicone we will be pouring will bond to the cured silicone if no release is applied. We could have also used Pattern Release 202, which is an aerosol version of petroleum jelly.

After assembling the mold frame, we align our pouring sprue and several vertical vents over our mold. All are attached to this wood bar, which has wood dowels on the ends to align and secure it in place. Then we make sure the sprue and each vent is lightly touching our model. Now we’re ready to pour the second half of our mold, again, using Rhodia V-330 silicone rubber.

The following day, we remove our vents, pull out the pouring sprue, and then demold our model. As you can see, the mold is perfect in accuracy and detail and ready to produce urethane castings.

We tape the two mold halves together with electrical tape and then form a small clay reservoir over our pouring sprue. This will enable us to have some extra material available to our mold in case it is needed when the pressure is applied.

Here we’re mixing our Freeman 1090 Clear polyurethane and adding one drop of red Freeman color tint. Next we pour the urethane into the mold until the clay reservoir is nearly full and then we place the mold in a pressure pot under 40 PSI of pressure. This pressure helps the urethane completely fill in the entire closed mold and thus minimize the possibility of voids in our casting.

When the urethane has cured, we demold our new part and the mold is ready for another casting.

We purposely did not polish our wax model in order to show the accuracy of our silicone rubber mold and the Freeman 1090 polyurethane. Our cast parts are perfect duplicates, showing every machining mark, just like our original model.

One of the advantages of machinable wax is that is can be easily polished to a very smooth surface using a razor knife and Freeman’s wax release. Casting this mouse in our mold would result in a smoother silicone rubber mold and a clearer, smoother urethane casting.